[Test] Cleaned up how a bunch of tests were ignored

Previously, many tests just had a return statement when the test shouldn't
be executed (e.g. Unix-only). This made it look like the test was successful
when in fact it wasn't executed at all.
Assert.Ignore() is now used instead to correctly exclude those tests.
Merge pull request #910 from akoeplinger/cleanup-test-ignores

[Test] Cleaned up how a bunch of tests were ignored